To define the Organization-Wide Defaults sharing settings, go to Setup | Security | Sharing Settings:
A selection of standard objects and all custom objects can be set in this page.
This OWD page is meant for setting the most restrictive access to a given object. This means that, from now on, you can ask an administrator to only grant additional access and not restrict it.
By default, Salesforce uses role hierarchies to grant access to records to the users that belong to roles above a given user hierarchy. This means that if a user owns a record (whose object is set as Private in the OWD, so it should be only visible to its owner), ...
